re-raise exception with full traceback

Rather than raising the exception using the local reference, use a bare
raise to ensure that the original traceback is preserved. This reports
the error as coming from inside the plugin code, rather than from inside
stevedore, and helps with debugging.

Change-Id: I4d7c002f378fb6491aa5c9e262084d70924112de

stevedore/driver.py

@@ -47,7 +47,7 @@ class DriverManager(NamedExtensionManager):
 
     @staticmethod
     def _default_on_load_failure(drivermanager, ep, err):
-        raise err
+        raise
 
     @classmethod
     def make_test_instance(cls, extension, namespace='TESTING',